1. Field of the Invention
The present invention relates to a semiconductor laser and a method for producing the semiconductor laser. More particularly, the present invention relates to a semiconductor laser suitable for optical fiber communication and the like, and a method for producing such a semiconductor laser.
2. Description of the Related Art
In order to improve the production yield of semiconductor lasers, a technique for growing a current blocking layer in addition to an active layer on a substrate by MOVPE (metal organic vapor phase epitaxy) has been studied. A semiconductor laser in which all semiconductor layers required for the semiconductor laser are grown on an InP single crystalline substrate by MOVPE has been reported. See the Journal of Crystal Growth 93 (1988) 792 A. W. Nelson et al.
Referring to FIG. 18, the conventional semiconductor laser is described. In FIG. 18, on a substrate 50, an InP buffer layer 51, an active layer 52, a p-InP first current blocking layer 53, an n-InP second current blocking layer 54, a p-InP third current blocking layer 55, and a contact layer 56 are formed. On the top face of the substrate 50, a p-side electrode 57 is formed. On the back face of the substrate 50, an n-side electrode 58 is formed. In this semiconductor laser, a current injected from the p-side electrode 58 is confined by the second current blocking layer 54 and then injected into the active layer 52.
In order to enhance the performance of semiconductor lasers, the study and development of semiconductor lasers having a single quantum well (SQW) structure and a multiquantum well (MQW) structure are actively pursued. The semiconductor laser having a quantum well type active layer can attain superior effects due to the quantum size effect of the active layer, as compared with a semiconductor laser having a bulk type active layer. For example, due to an increase of the differential coefficient, a reduction of TM light-emission, and the like, the semiconductor laser oscillates at a low threshold level, so that a large light output can be obtained with high efficiency. In addition, due to an increase of the attenuation oscillating frequency and a reduction of the amplitude increasing coefficient, the response speed is increased and the chirping is reduced.
In order to further increase the differential coefficient, a "graded doped structure" is proposed. In the graded doped structure, p-type impurities are doped in a part of the barrier layer. Especially when a strained quantum well type active layer has the graded doped structure, the performance of the semiconductor laser is expected to be improved.
FIG. 19 shows a semiconductor laser provided with a quantum well type active layer having the conventional graded doped structure (K. Uomi, T. Mishima, N. Chinione, Jpn. J. Appl. Phys., 29(1990)88). In the semiconductor laser shown in FIG. 19, on a GaAs substrate 61, an n-type GaAs buffer layer 62, an n-type InAlAs cladding layer 63, a graded doped quantum well layer 68 which is sandwiched between non-doped GRIN-GaAlAs layers 64, a p-type GaAlAs cladding layer 69, an n-type GaAs current blocking layer 70, an oxide film 71, and a p-side electrode 73 are successively deposited in this order. The oxide film 71 is provided with a stripe-shaped opening. The p-side electrode 73 is in contact with a part of the n-type GaAs current blocking layer 70 through the opening. At the contact portion, a Zn diffused region 72 extending to the p-type GaAlAs cladding layer 69 is provided. On the back side of the substrate 61, an n-side electrode 74 is provided.
In the above-described conventional semiconductor laser (FIG. 18), it is confirmed by a long-term high-temperature and acceleration test (FIG. 20) that the driving current required for obtaining a predetermined light output level is not varied but stable. However, in an initial stage of the test, an increase of driving current caused by the increase of the threshold level of laser oscillation is observed. When a semiconductor laser is intended for practical use, the increase of the driving current must be avoided even in the initial stage.
In the case where a current blocking layer is formed by LPE (liquid phase epitaxy), a method for suppressing the increase of the threshold level is established. On the other hand, in the case of the MOVPE, there still exists the problem that the threshold current is increased.
In addition, the conventional semiconductor laser having a quantum well type active layer with a graded doped structure (FIG. 19) involves a problem in that the performance thereof is not so much improved, as compared with the case without the graded doped structure. This is because in the crystal growth after the step of forming the graded doped structure, the dopant is disadvantageously diffused from the semiconductor layer which is grown thereon to the active layer. Thus, the graded doped structure disappears.